Download What is an Earthquake?

Absolute-Dating Methods-4 types
• The discovery of radioactivity
– destroyed Kelvin’s argument for the age of Earth
– and provided a clock to measure Earth’s age
• A. Radioactivity is the spontaneous decay
– of an atom’s nucleus to a more stable form
• The heat from radioactivity
– helps explain why the Earth is still warm inside
• Radioactivity provides geologists
– with a powerful tool to measure
– absolute ages of rocks and past geologic events
B. Fission Track Measurements (0.4 to 1.5 MYA)
C. Carbon 14 dating- less than 70,000 years old
D. Tree Ring dating- back as much as 14,000 years ago
Radioactive Decay
• The different forms of an element’s atoms
– with varying numbers of neutrons
– are called isotopes
• Different isotopes of the same element
– have different atomic mass numbers
– but behave the same chemically
• Radioactive decay is the process whereby
– an unstable atomic nucleus spontaneously changes
– into an atomic nucleus of a different element
• Three types of radioactive decay:
– In alpha decay, two protons and two neutrons
– (alpha particle) are emitted from the nucleus.
Radioactive Decay
– In beta decay, a neutron emits a fast moving
electron (beta particle) and becomes a proton.
– In electron capture decay, a proton captures an
electron and converts to a neutron.
Radioactive Decay
• Some isotopes undergo only one decay step
before they become stable.
– Examples:
• rubidium 87 decays to strontium 87 by a single beta
emission
• potassium 40 decays to argon 40 by a single electron
capture
• But other isotopes undergo several decay steps
– Examples:
• uranium 235 decays to lead 207 by 7 alpha steps and 6
beta steps
• uranium 238 decays to lead 206 by 8 alpha steps and 6
beta steps
Uranium 238 decay
Concept of ‘Half-Lives’
• The half-life of a radioactive isotope
– is the time it takes for one half
– of the atoms of the original unstable parent isotope
- to decay to atoms
– of a new more stable daughter isotope
• The half-life of a specific radioactive isotope
– is constant and can be precisely measured
• The length of half-lives for different isotopes
–
–
–
–
of different elements
can vary from
less than 1/billionth of a second
to 49 billion years
• Radioactive decay
– is geometric not linear,
– so has a curved graph
Geometric Radioactive Decay
– In radioactive
decay,
– during each
equal time unit
• one half-life,
– the proportion
of parent atoms
– decreases by 1/2
Determining Age
• By measuring the parent/daughter ratio
– and knowing the half-life of the parent
• which has been determined in the laboratory
– geologists can calculate the age of a sample
– containing the radioactive element
• The parent/daughter ratio
– is usually determined by a mass spectrometer
• an instrument that measures the proportions
• of atoms with different masses
Determining Age
• For example:
– If a rock has a parent/daughter ratio of 1:3
= a parent proportion of 25%,
– and the half-life is 57 million years,
• how old is the rock?
– 25% means it is 2 halflives old.
– the rock is 57 x 2 =114
million years old.
What Materials Can Be Dated?
• Most radiometric dates are obtained
– from igneous rocks
• As magma cools and crystallizes,
– radioactive parent atoms separate
– from previously formed daughter atoms
• Because they fit, some radioactive parents
– are included in the crystal structure
– of certain minerals
• The daughter atoms are different elements
–
–
–
–
with different sizes
and, therefore,
do not generally fit
into the same minerals as the parents
• Geologists can use the crystals containing
– the parents atoms
– to date the time of crystallization
Igneous Crystallization
• Crystallization of magma separates parent atoms
– from previously formed daughters
• This resets the radiometric clock to zero.
• Then the parents gradually decay.
Not Sedimentary Rocks
• Generally, sedimentary rocks cannot be
radiometrically dated
– because the date obtained
– would correspond to the time of crystallization of
the mineral,
– when it formed in an igneous or metamorphic rock,
– not the time that it was deposited as a sedimentary
particle
• Exception: dating the mineral glauconite,
– because it forms in certain marine environments as
a reaction with clay
– during the formation of the sedimentary rock
Sources of Uncertainty
• In glauconite, potassium 40 decays to argon 40
– because argon is a gas,
– it can easily escape from a mineral
• A closed system is needed for an accurate date
– that is, neither parent nor daughter atoms
– can have been added or removed
– from the sample since crystallization
• If leakage of daughters has occurred
– it partially resets the radiometric clock
– and the age will be too young
• If parents escape, the date will be too old.
• The most reliable dates use multiple methods.
Sources of Uncertainty
• During metamorphism, some of the daughter
atoms may escape
– leading to a date that is too young.
– However, if all of the daughters are forced out
during metamorphism,
– then the date obtained would be the time of
metamorphism—a useful piece of information.
• Dating techniques are always improving.
– Presently measurement error is typically <0.5% of
the age, and even better than 0.1%
– A date of 540 million might have an error of ±2.7
million years or as low as ±0.54 million
Dating Metamorphism
Dating the whole rock
yields a date of 700
million years = time of
crystallization.
a. A mineral has just
crystallized from magma.
b. As time passes, parent
atoms decay to daughters.
c. Metamorphism drives
the daughters out of the
mineral as it
recrystallizes.
d. Dating the mineral today
yields a date of 350
million years = time of
metamorphism, provided
the system remains closed
during that time.
Long-Lived Radioactive
Isotope Pairs Used in Dating
• The isotopes used in radiometric dating
– need to be sufficiently long-lived
– so the amount of parent material left is measurable
• Such isotopes include:
Parents
Daughters
Half-Life (years)
Uranium 238
Uranium 234
Thorium 232
Rubidium 87
Potassium 40
4.5 billion
704 million
14 billion
48.8 billion
1.3 billion
Lead 206
Lead 207
Lead 208
Strontium 87
Argon 40
Most of these
are useful for
dating older
rocks
2. Fission Track Dating
• Uranium in a crystal
– will damage the crystal structure as it decays
• The damage can be seen as fission tracks
– under a microscope after etching the mineral
• The age of the
sample is related to
– the number of
fission tracks
– and the amount of
uranium
– with older samples
having more tracks
• This method is
useful for samples
between 1.5 and
0.04 million years
old
3. Radiocarbon Dating Method
• Carbon is found in all life
• It has 3 isotopes
– carbon 12 and 13 are stable but carbon 14 is not
– Carbon 14 has a half-life of 5730 years
– Carbon 14 dating uses the carbon 14/carbon 12 ratio
• of material that was once living
• The short half-life of carbon 14
– makes it suitable for dating material
– < 70,000 years old
• It is not useful for most rocks,
– but is useful for archaeology
– and young geologic materials
Carbon 14
• Carbon 14 is constantly forming
– in the upper atmosphere
• The carbon 14 becomes
– part of the natural carbon cycle
– and becomes incorporated into
organisms
• While the organism lives
–
–
–
–
it continues to take in carbon 14
but when it dies
the carbon 14 begins to decay
without being replenished
• Thus, carbon 14 dating
– measures the time of death
4. Tree-Ring Dating Method
The age of a tree can be determined by counting the annual growth rings
-in lower part of the stem (trunk)
The width of the rings are related to climate and can be correlated from tree to tree
-a procedure called cross-dating
-The tree-ring time scale now extends back 14,000 years
•
In cross-dating, tree-ring patterns are used from different trees, with overlapping life spans
Chp 9: Earthquakes
What is an Earthquake?
Definition: a sudden motion or trembling of the Earth caused by the abrupt release
of slowly accumulated elastic energy in rocks.
A. Stress
-The force that is exerted against an object; the rocks of the Earth’s crust are
stressed by tectonic forces
-Whenever an object is stressed it changes its size and shape
B. Strain- The deformation that results from stress
1. elastic deformation
-Stress applied slowly
-When stress its removed the object springs back to its original size and shape
2. All rocks deform elastically when tectonic stress is applied; the sudden release
of that stored elastic energy when the rock fractures causes earthquakes
3. elastic limit: the limit beyond which the rock cannot deform elastically
4.plastic deformation: when a rock will not return to its original shape when the
stress is released
5. brittle rupture-When a rock is deformed beyond the elastic limit, it may ruptu
It breaks sharply and the fracture becomes a permanent feature of the rock
Chapter 9-Earthquakes:
Izmit, Turkey 1999:
est 17,000 people died, 240,000 buildings damaged
Fig. 9-CO, p.240
Table 9-1, p.242
Chp 9: Earthquakes
C. Deformation of Tectonic Plate
1. Segmented lithosphere that moves relative to each other by gliding over the
asthenosphere
2. As the plates move they slip past one another along immense fractures that form
the boundaries between adjacent plates
3. The slippage is not smooth and continuous, but occurs as rapid jerks as one plate
suddenly slips, causing an earthquake.
EARTHQUAKES, FAULTS, AND TECTONIC PLATES
A. Fault
1. A fracture in rock along which movement has occurred in the past
2. Earthquakes start on old faults that have moved many times in the past and will
move again in the future.
3. If new stresses develop in a region of the crust, new faults form and earthquakes
can occur in places that were previously earthquake free.
4. fault creep
a. Slow movement of plate past one another at a continuous snail-like pace
b. The movement occurs without violent and destructive earthquakes
c. seismic gap: fuzzy area on graph- little or no seismic activity
Inactive fault- pink rocks on left against white rocks on right
p.266b
Light reflecting off rock surface- fault plane, slickensided…
p.266a
Chp 9: Earthquakes
B. Tectonic Plates
1. Most earthquakes occur along the faults separating tectonic plates
2. Less commonly, earthquakes occur where thick piles of sediment
have accumulated
a. Causes of earthquakes in plate interiors are not as well
understood
b. Some interior quakes occur where thick piles of sediment
have accumulated
c. Other interior quakes may be caused by plate movements
d. Rebound: “ Isostasy” over central US because of glacial
covering
Relationship between earthquake epicenters and plate boundaries!!
approx 80% occur within circum-Pacific. Each dot represents one epicente
Fig. 9-4, p.247
Chp 9: Earthquakes
EARTHQUAKE WAVES
A. Body Waves
1. Seismic Waves
-waves that travel through rock
-initiated naturally by earthquakes
-can also by produced artificially by explosive charges detonated on or beneath
the Earth’s surface
seismology: the study of earthquakes and of the structure of the Earth’s interior
from evidence provided by seismic waves
body waves
-travel through the interior of the Earth
-start from the initial rupture point, or focus, of an earthquake.
-Epicenter: point on the Earth’s surface directly above the focus
a. primary wave- also called p-wave
-formed by alternate compression and expansion of the rock—like a slinky.
-Transmitted in both liquids and solids
-Travels at speeds of 5-7 km/sec in the crust and 8 km/sec in the upper mantle
-The first wave to reach an observer.
b. Secondary wave--also called S-wave
-Form when shearing forces are transmitted
-Travel at speeds between 3-4 km/sec in the crust
-Move only through solids
Rocks are deformed as high energy from earthquake passes throughrocks are deformed, store energy, then bend as a result. When internal
strength is exceeded, rocks fracture-causing an earthquake. They
rebound to their original shape. b) fence in Marin County Ca, offset 5m
Fig. 9-1, p.243
Surface waves due to earthquake
Pwave= parallel to direction of movement
Swave= perpendicular
Fig. 9-8, p.251
Chp 9: Earthquakes
5. Secondary wave--also called S-wave
-Form when shearing forces are transmitted
-Travel at speeds between 3-4 km/sec in the crust
-Move only through solids
B. Surface waves
1. L-Waves: “Surface Waves”
-Two types of L-waves
a. Up-and-down motion
b. Side-to-side vibration
Surface waves cause the most property damage because of the ground motion
associated with them
C. Measurement of seismic waves
-seismograph: a device that graphs seismic waves
-seismogram: the record of an earth vibration
An earthquake measuring station generally has at least three seismographs
-two horizontal seismographs
a. oriented to measure east-west movements
b. oriented to measure north-south movements
-one vertical seismograph
Focus of
earthquake:
Where rupture
begins and
energy is
released.
The location on
the surface of the
earth, vertically
above the focus is
known as the
Epicenter.
Seismic waves move
out in all directions…
Fig. 9-3, p.247
Types of S waves: Rayleigh and Love waves:
a. Rayleigh: move material in elliptical path parallel to wave direction
b. Love: move material back and forth in a horizontal plane, perpendicular to the motion of the earthquake wave. VERY dangerous to homes
Fig. 9-9, p.251
Fig. 9-9abc, p.251
Chp 9: Earthquakes
LOCATING THE SOURCE OF AN EARTHQUAKE
Time-travel Curves
1. Graphs used to quantify the general relationship between distance from an
earthquake epicenter and arrival times of the different types of quakes
2. to create a time travel graph one must know both when and where the earthquak
occurred
3. graph can then be used to measure the distance between a recording station and
earthquake whose epicenter is unknown
EARTHQUAKES AND HUMANS
A. Measurements of earthquake strength
1. Mercalli Scale: a qualitative scale of earthquake intensity and measures the
effects of an earthquake at a particular place
2. Richter scale
a. A quantitative scale of earthquake magnitude based on measurements
made by a seismograph
b. first refined by Charles Richter in 1935
c. The magnitude is determined by measuring the amplitude of the largest wave
recorded by a seismograph.
Adjustments are made for the distance from each recording station to the earthquake
Table 9-2, p.254
Modern seismographs record earthquake
waves electronically-see a.
b. A horizontal motion seismograph.
c. A vertical motion seismograph.
Fig. 9-2, p.246
The Richter magnitude scale
measures the amount of energy
released by an earthquake at its
source.
1.Measure maximum amplitude
of largest seismic wave, mark on
right hand scale.
2. Difference in arrival time of
P and S waves, in seconds, is
marked on left hand scale.
3. Draw a line between the two
points-magnitude of earthquake
is where the line crosses center
scale.
Fig. 9-14, p.257
Chp 9: Earthquakes
d. Scale is logarithmic
1.An increase of one unit on the scale represents a 10-fold increase in
the amplitude of a recorded earthquake wave
2.An increase of one unit on the scale corresponds approximately to a
30-fold increase in energy related during the quake
B. Earthquake Damage
1. Ground motion
-Motion of ground is dependent upon the rock and the soil type
- Some types of building materials and design features are more able to
withstand an earthquake than others
2. Permanent alteration of landforms
Scarps: a short, steep cliff formed by the vertical displacement of land by an
earthquake (ex: Balcones Escarpment)
3. Fire
4. Landslides
5. Tsunamis: Seismic sea wave caused by displacement of the sea floor
San Francisco, Marina district-fires caused by ruptured gas lines...
Fig. 9-18, p.261
a. fault scarp: block on right moved
up with respect to one at bottom
b. Earthquake
triggered landslide
,seen in distance,
which dammed
lake
Fig. 9-22, p.268
30,000 people were killed in 1993 quake in India
Fig. 9-17b, p.261
Earthquake damage in Pacific: Kobe Japan; Oakland, Ca; Northridge
Fig. 9-6, p.249
Tsunami crashes into street in Hilo, Hawaii in 1946-caused by
Earthquake in Aleutian Islands….159 people died in Hilo
Fig. 9-19, p.264
Chp 9: Earthquakes
EARTHQUAKE PREDICTION
Long-Term Prediction
1. Motion along a fault
a. On segments of the fault where fault creep occurs, the plates slip past
one another smoothly and without major earthquakes
b. In other segments of the fault, the plates “hop” past one another in a
series of small jumps causing numerous, small, non-damaging earthquakes
c. In still other segments of the fault, plates become locked for tens to
hundreds of years and then produce catastrophic earthquakes when they
break free.
2. Seismic gap
a. An immobile region of a fault bounded by moving segment
b. Rock within the seismic gap is accumulating elastic deformation and will
eventually fracture producing a major earthquake
3. Historical studies of earthquake activity make it relatively easy to
identify zones of high earthquake hazard.
Earthquake predictions-look for
gaps in activity….
Fig. 9-24, p.269
3 gaps evident in seismic activity along San Andreas fault
Chp 9: Earthquakes
B. Short-Term Prediction
1. A reliable early warning system; a signal or group of signals that immediately
precedes an earthquake
2. Foreshocks
-Small earthquakes that precede a large quake by an interval ranging from a
few seconds to a few weeks
-Only about ½ of the major earthquake sin recent years were preceded by a
significant number of foreshocks
-Measure change in the shape of the land; distortions of the crust may precede majo
earthquakes.
3. release of radon preceding an earthquake
4. strange animal behavior preceding an earthquake
C. Social and Economic Factors in Earthquake Prediction
Short-term earthquake prediction is not only a formidable scientific problem but it
also involves political, social, and economic issues
3 seismograph stations are
needed to locate the epicenter
of any earthquake.
Procedure is as follows:
1. P-S time interval plotted
on time distance graph for
each station. This
determines the distance
each station is from the
epicenter.
2. A circle with that radius
is drawn from each station,
the intersection of the
3 circles locates
the epicenter.
Fig. 9-11, p.253
Seismogram showing the
a. arrival order and pattern of
different waves.
b. Seismogram for 1906 San
Francisco earthquake. Recorded
in Germany.
c. A time-distance graph showing
the average travel times for P and
S waves
Fig. 9-10, p.252
Mercalli intensity effects during
Northridge earthquake
Fig. 9-12, p.255
Comparison between the
general geology of the
San Francisco area (a)
and the Mercalli Intensity
map of same area (b).
A close correlation exists
between geology and intensity.
a. Areas underlain by bedrock
had lowest intensity.
b. Areas underlain by bay mud
or artificial fill were shaken
most violently.
Fig. 9-13, p.256
Table 9-3, p.257
Amplitude and duration of earthquake waves changes as pass from bed
rock into unconsolidated materials.
Fig. 9-15, p.260
Things you can do to strengthen your home-attach to foundation…..
p.259
Effects of ground shaking on water saturated soil-building collapse, Japan
Fig. 9-16, p.260
Pacific Tsunami early warning system: prediction system
Fig. 9-21, p.267
Man-made earthquakes??
a. Waste water injection well in
Colorado
b. Average number of earthquakes
in Denver area.
c. Volume of fluid injected into
well.
Note-since injection has stopped,
no earthquakes generated….fluid
‘greased’ fault zones, aiding
quake generation???....
Fig. 9-26, p.272
Chp 9: Earthquakes
STUDING THE INTERIOR OF THE EARTH
A. Seismic Tomography
1. Geologists use seismic waves to study the interior of the Eart
2. Properties of waves
a. In a uniform homogeneous medium, a wave radiates velocity
b.The velocity of a seismic wave depends on the nature of the material that
travels through; i.e. its rigidity and density
c. When waves pass form one material to another, they refract (bend)
d. P-waves are compressional waves and travel through all media—gases,
liquids, and solids—whereas S-waves are only transmitted through solids.
3. In a uniform and homogeneous Earth waves will radiate form the focus of an
earthquake in concentric spheres and travel uniformly through the planet. Uniform
propagation of waves is not observed
B. Discovery of the core
1. Shadow Zone
a. No direct waves are defected beyond 105 degrees from the focus
b. Caused by a discontinuity deep in the interior of the Earth
c. The shadow zone exists because of refraction of the p-waves at the mantle-core
boundary and the failure of s-waves to pass through
Chp 9: Earthquakes
2. The failure of s-waves to pass through the outer core, and the refraction of p-waves
shows that the core is composed of an inner solid sphere surrounded by an outer liquid
sphere
C. Discovery of the Crust-Mantle Boundary
Mohorovicic Discontinuity (Moho): the boundary between the crust and the mantle
was first identified as the boundary between different types of rock that transmit waves
at different velocities.
THE EARTH’S INTERIOR
Seismic Tomography: The study of the Earth’s interior indirectly by studying the
action of seismic waves.
This information was discovered by seismic interpretation
A. The Crust
1. The outer shell of the Earth
2. Oceanic crust
-7-10 km thick
-p-waves travel through oceanic crust (basaltic composition) at 7 km/sec
3. Continental crust
-20-70 km thick
-p-waves travel through continental crust (granitic composition) at 6km/sec
Chp 10: Earth’s Interior
Chp 9: Earthquakes
B. The Mantle
1. 2900 km thick and comprises about 80% of the volume of the Earth
2. Large quantities of basalt magma originate in the mantle
3. The layers within the mantle
a.Upper mantle
--Extends from the base of the curst downward to about 670 km beneath the surfa
--composed primarily of peridotite
-- subdivided into three layers
1. lithosphere—crust and uppermost part of the mantle—where most
earthquakes occur
2. asthenosphere – extends from the base of the lithosphere to a depth of 350
km
3. low-velocity layer—separates lithosphere form asthenosphere
C. Outer Core – known to be liquid because of the behavior of seismic waves. It is
speculated that the material is liquid iron and nickel.
D. Inner Core – known to be solid because of the behavior of seismic waves and is
thought to be solid iron and nickel.
Chp 9: Earthquakes: Summary
Earthquakes – causes of ….and locations of…
a. release of accumulated stress along faults
b. commonly occur along plate boundaries: circum-Pacific and
Mediterranean region
Earthquakes: location determined bya. minimum of 3 operating seismograph stations
b. measure travel time to each station, plot distance as radius of circle
c. where 3 circles intersect is the Epicenter-the location on the surface
of the Earth directly above earthquake center
Earthquake energy transmitted as WAVES…various types of waves:
a. P waves: primary, distort grains parallel to motion of wave
b. S waves; secondary, distort grains perpendicular to motion of wave
Tonga volcanic arc, Pacific O: focal depth increases in well-defined
Zone that defines subducting oceanic plate…called Benioff Zone….
Subducting oceanic plate…
Fig. 9-5, p.248
Seismic hazard map-showing peak ground accelerations. The
higher the number the greater the hazard. Note relationship with
plate boundaries-circum Pacific and Mediterranean-Asian
Fig. 9-23, p.268
Chp 9: Earthquakes: Summary
Types of S waves: (surface)
a. Rayleigh waves: material distorted in elliptical path
b. Love waves: side to side motion
Richter Scale:
a. an increase of 1 unit on scale represents 10 fold increase in amplitude
b. an increase of 1 unit on scale represents 30 fold increase in energy
Seismic gap: useful for prediction today (not accurate)
a. An immobile region of a fault bounded by moving segments
b.Rock within the seismic gap is accumulating elastic deformation and w
eventually fracture producing a major earthquake
Other predictive tools:
a. foreshock swarm intensity increases dramatically
b. radon gas increase
c. animal behavior
Chp 9: Earthquakes: Summary
Propogation of seismic waves from earthquakes has greatly aided our
understanding of composition of interior of Earth:
A. The Crust
1. The outer shell of the Earth
2. Oceanic crust
-7-10 km thick
-p-waves travel through oceanic crust (basaltic composition) at 7 k
3. Continental crust
-20-70 km thick
-p-waves travel through continental crust (granitic composition) at 6km/sec
B. The Mantle
1. 2900 km thick and comprises about 80% of the volume of the Earth
2. Large quantities of basalt magma originate in the mantle
3. The layers within the mantle
Chp 9: Earthquakes: Summary
a.Upper mantle
--Extends from the base of the curst downward to about 670 km beneath the surface
--composed primarily of peridotite
-- subdivided into three layers
1. lithosphere—crust and uppermost part of the mantle—where m
earthquakes occur
2. asthenosphere – extends from the base of the lithosphere to a de
km
3. low-velocity layer—separates lithosphere form asthenosphere
C. Outer Core – known to be liquid because of the behavior of seismic waves. It is
speculated that the material is liquid iron and nickel.
Inner Core – known to be solid because of the behavior of seismic waves and is
thought to be solid iron and nickel.
Chp 10: Earth’s Interior
Fig. 9-1b, p.243
Charleston, SC earthquake of 1886…largest registered in eastern US
Fig. 9-7, p.250
Falling structures account for most deaths in quakes
Fig. 9-17a, p.261
Artist’s rendition of tsunami
that hit Anchorage, Alaska
after earthquake in 1964
Fig. 9-20, p.265
Earthquake monitoring and prediction:
dilatancy=cracking
Fig. 9-25, p.270
Table 9-4, p.271
p.244
p.245
Fig. 9-1a, p.243
Fig. 9-9d, p.251